Wheel bearing apparatus that freely rotationally supports a wheel of a vehicle includes a wheel hub to mount the wheel via a double row rolling bearing. There are apparatus for both driving wheels and driven wheels. For structural reasons, an inner ring rotation type is used for the driving wheels and both the inner ring rotation type and the outer member rotation type are used for the driven wheels. There are four generation types of wheel bearing apparatus. A first generation type has a wheel bearing that includes a double row angular contact ball bearing etc. fit between a knuckle, forming part of a suspension apparatus, and a wheel hub. A second generation type has a body mounting flange or a wheel mounting flange directly formed on the outer circumference of an outer member. A third generation type has one inner raceway surface directly formed on the outer circumference of a wheel hub. A fourth generation type has an inner raceway surfaces formed on the outer circumferences, respectively, of a wheel hub and an outer joint member.
Angular contact ball bearings are known that included an inner ring and an outer ring (outer member) formed by pressing a steel plate. For example, FIG. 11 shows an angular contact ball bearing 50 used in a magnetic hard disc device. An outer ring 51 and a pair of inner rings 52, 53 are formed from stainless steel by a pressing or rolling process.
The outer ring 51 is formed at substantially its axially central portion with an annular projection 51a projecting radially inward from its inner circumference. Outer raceway surfaces 51b, 51c are formed at either side of the annular projection 51a. The outer ring 51 is fit into an aperture of a housing 54 and is axially positioned with a flange 51d, formed at its one end, abutted against the end face of the housing 54. An annular recess 51e is formed on the outer circumference of the outer ring 51 by the void left by forming the annular projection 51a on the inner circumference of the outer ring 51. The annular recess is filled with adhesive to secure the outer ring 51 in the aperture of the housing 54.
On the other hand, the inner rings 52, 53 are fit into the outer ring 51. Axially outer ends of the inner rings 52, 53 are formed with curved shoulders 52a, 53a, respectively. The curved shoulders 52a, 53a are formed with inner raceway surfaces 52b, 53b, respectively. Double row balls 56, 56 are arranged between the inner raceway surfaces 52b, 53b, of the inner rings 52, 53 and the double row outer raceway surface 51b, 51c, of the outer ring 51. The balls 56, 56 are held by cages 57, 57 in each row.
Fitting portions 52c, 53c, clearance-fit onto a shaft member 55, are formed on the inner circumferences of the inner rings 52, 53, respectively. After the inner rings 52, 53 have been clearance-fit onto the shaft member 55, a cylindrical weight 59, with a constant weight, is laid on the curved shoulder portion 52a of one inner ring 52. The curved shoulder portion 53a of the other inner ring 53 is pressed against the flange portion 55a of the shaft member 55, via balls 56 by weight 59. Thus, a suitable pre-load is applied to the angular contact ball bearing 50.
After completion of the clearance-fit, a gap between the curved shoulder 52a of one inner ring 52 and the shaft member 55 is filled with adhesive 60. Thus, one inner ring 52 is prevented from slipping off from the shaft member 55 by the adhesive. The other inner ring 53 is prevented from slipping off from the shaft member 55 by its flange 55a. (See, Japanese Laid-open Utility Model Publication No. 1835/1994).
If the angular contact ball bearing 50 is used as a wheel bearing where the outer member 51 and the inner rings 52, 53 are formed from steel plates by a pressing process, it is necessary to mount seals in the annular openings formed between the outer ring 51 and the inner rings 52, 53. This not only increases the number of parts, but significantly modifies the structure to provide spaces for the seals. Thus, the manufacturing cost, weight and size are increased. In addition, it is difficult to have the same accuracy of components, more particularly, of each raceway surface, as that of components made a by cutting process of the prior art.
In addition, it is believed that the contact ellipse of the ball 56 tends to ride over a shoulder of the outer raceway surface 51b and get out of the bearing (so-called “riding over of shoulder”). This is caused by a moment load applied to the wheel bearing during turning of a vehicle. It is further believed that an edge load would be caused at the shoulder A by the riding over of shoulder. The term “edge load” means an excessive stress concentration caused in a corner etc. This is one phenomenon that causes a premature flaking.
During forming of the annular projection 51a, by recessing the axially central portion of the outer ring 51 during the rolling process, if the height of the annular projection 51a, from the outer raceway surface 51b is made large, it is difficult for the ball 56 to ride over the shoulder A during the turning moment when a moment load is applied to the bearing. However, in such a case, it is believed that the wall thickness at a transition portion B between the outer circumference and the annular recess 51e would be thinned depending on the radius of curvature R. Thus, cracks would be caused in the shoulder portion A when the height of shoulder is large. It will be appreciated that the generation of cracks will be prevented by suppressing the drawing ratio while increasing the number of machining steps. This not only increases the manufacturing costs but reduces the manufacturing accuracy and makes it difficult to assure a desirable bearing accuracy.